The potential of Secondary Ion Mass Spectrometry (SIMS) in the analysis of solid samples such as minerals has long been recognized, and the geochemical applications have been discussed by J. F. Lovering in NBS Spec. Publ. 427, 135-178 (1975) and N. Shimuzu and S. R. Hart in Ann. Rev. Earth Planet Sci. 10, 483-526 (1982). The ability to cover virtually the complete periodic table, coupled with excellent sensitivity for many elements and high spatial resolutions, should give the technique wide application in the characterization and elemental analysis of complex solids such as minerals. The high signal/background ratios inherent in mass spectrometry coupled with excellent ion yields make it possible to attain detection limits of parts per billion for a number of elements, as set out by S. J. B. Reed in Scanning 3, 119-127 (1980).
As shown in FIG. 1, in this method a high energy "primary" ion beam such as .sup.16 O.sup.- impinges on the sample of interest, and atoms and ions are ejected from the sample. Normally positive ions from the sample (at 4500 V in the CAMECA instrument) are accelerated towards an immersion lens, and the ions are then mass analyzed. A typical mass spectrum of a gold coated sphene is shown in FIG. 2. This spectrum indicates that almost all the elements in the periodic table can be analyzed, and that the intensities (and thus sensitivities) are excellent. Also the spatial resolution is high, down to 5 microns in this instrument. However, minor and trace elements cannot be easily detected because of molecular ions which have the same nominal mass as atomic species. Thus, for example, .sup.40 Ca.sup.16 O.sup.+ overlaps with .sup.56 Fe.sup.+, and the oxides of the light rare earth elements overlap with the atomic ions of the heavy rare earths.
Thus the potential of SIMS has yet to be realized, perhaps with the exception of lead isotope determinations which have been used with success in dating lunar and terrestrial samples, as disclosed by C. A. Anderson and J. R. Hinthorne in Earth and Planetary Science Letters 14, at 195-200 (1972); R. W. Hinton and J. V. P. Long in Earth and Planetary Science Letters 45, at 309-325 (1979); and W. S. Meddaugh, H. D. Holland and N. Shimuzu in Ore Genesis, The State of the Art, edited by G. C. Amstutz et al (Springer-Verlag 1982). The signal/background advantage is often lost in a slough of molecular ion peaks (metal oxides, hydrides and hydroxides) which are part of the mass spectrum. (FIG. 2) This is an acute problem in geological specimens which typically contain a large number of elements at both major and trace levels. These samples are commonly insulators, or semiconductors at best, and surface charging has introduced further limitations in using the ion beam for analysis. Charging can result in distortion and instability of the primary beam, and is typically minimized using a negatively charged primary beam, conductive coatings (as set out A. Lodding, S. J. Larsson and H. Odelins in Z. Naturforsch 339 at 697-708 (1978)), low primary beam currents (as set out by M. G. Dowsett, R. M. King and E. H. C. Parker in Surface Science 71 at 541-547 (1978)), or by placing metal grids over the specimen surface (as set out by G. S. Slodzian in Ann. Phys. 9 at 591 (1964)). Werner and Morgan in J. Appl. Phys. 47, 1232-1242 (1976) proposed that a small conductive aperture set above the insulating specimen surface could confine and stabilize charging. Experimental proof of this concept has been provided by our recent studies.
Two methods have been developed to reduce the molecular interference problem. If very high mass resolution can be obtained, as in the CAMECA instrument, molecular ions such as .sup.40 Ca.sup.16 O.sup.+ can be distinguished from .sup.56 Fe.sup.+ on the basis of mass defects. However, the resulting intensity loss is so large that this method cannot be generally used. A more generally applicable approach has been the use of "kinetic energy analysis" (KEA) of the secondary ions to suppress molecular species. The kinetic energy distributions of atomic and molecular ions usually differ significantly within the lowest 100 eV of energy distribution (FIG. 3), with the molecular ions decreasing in intensity relative to atomic ions at higher kinetic energies. Thus by biasing the sample to +4400 V (instead of +4500 V) and employing a narrow energy window, the molecular ion/atomic ion contribution can be often decreased considerably. In FIG. 4(a), the CrO.sup.+ /Cr.sup.+ ratio is 1.times.10.sup.-3, whereas it is close to 1 in a normal spectrum. However, this kinetic energy selection procedure also decreases intensities by 1-2 orders of magnitude, and the molecular ion peaks are often still too large for trace element analysis; in FIG. 2, for example, the complete rare earth elements cannot be analyzed.
Thus, even with the present CAMECA instrument, which is the best one marketed in the world at the present time, complete analysis of complex solids (metals, alloys, semiconductors, minerals, glasses, plastics) cannot be performed. This has necessitated the development of much more expensive instruments such as that of the Isotrace project at the University of Toronto where a multi-million dollar accelerator is used to break up molecular ions.
A number of patents have been addressed to improving the effectiveness of spectrum analysis:
Canadian Pat. No. 995,825 to Brongersma is addressed to an ion scattering spectrometer for analyzing the surface layer of a material, wherein the device has means to produce a primary, substantially mono-energetic ion beam, deflection means to direct a primary ion beam onto the surface layer of the sample, a diaphragm aperture to pass the ions which are scattered at a predetermined angle relative to the axis of the primary ion beam at the surface layer, and an electrostatic analyzer and ion detector to determine the kinetic energy of the scattered ions passing through this diaphragm. The primary ion beam is deflected along the axis of the analyzer via apertures in two coaxial cylindrical electrodes. The diaphragm aperture is substantially annular and coaxial with the analyzer and is positioned to pass ions which are scattered over an angle exceeding 90.degree..
Canadian Pat. No. 1,015,467 to Brongersma and Walinga discloses an ion scattering spectrometer somewhat similar to the aforementioned device but having the ability to perform structure analysis as well as mass analysis of the surface layer. The device has means for generating a primary substantially mono-energetic ion beam, a diaphragm aperture for passing a secondary ion beam to be analyzed, and an electrostatic analyzer having a detector for determining the kinetic energy of the ions of the secondary ion beam. The electrostatic analyzer comprises two substantially cylindrical coaxial electrode, and the diaphragm aperture is substantially annular and coaxial with the analyzer. The detector comprises a large number of individual detector elements arranged in a ring which is substantially coaxial with the analyzer.
Canadian Pat. No. 996,685 to Van Nieuwland et al. discloses an ion scattering spectrometer wherein there are provided means to produce a primary substantially monoenergetic ion beam, a diaphragm aperture for passing ions which are scattered at the surface layer at a previously determined angle with respect to the axis of the primary ion beam, and an electrostatic analyzer and a detector to determine the kinetic energy of the scattered ions passed through the diaphragm. The detector is annular and the primary ion beam is directed through the aperture in the centre of the detector.
Canadian Pat. No. 1,021,882 to Erickson and Smith discloses an ion scattering spectrometer utilizing a charge exchange process, wherein the composition of a surface is determined by measuring the loss of kinetic energy as a result of binary scattering.
Canadian Pat. No. 1,048,163 to Le Gressus et al discloses a process of an apparatus for elementary and chemical analysis of the energy of secondary electrons emitted by the sample when the sample is exposed to a monoenergetic beam of primary electrons concentrated on the surface of the sample (i.e., the examination of a sample by the spectrum of Auger emission). The intensity of a beam of monoenergetic primary electrons emitted by an electron gun is modulated according to a sinusoidal law. The secondary electrons emitted by the sample are collected and the intensity of the collected beam is detected.
Canadian Pat. No. 943,670 to Goff discloses an ion scattering spectrometer capable of the surface analysis of insulating materials. Ion generating means produce a monoenergetic beam of primary ions and an energy analyzer receives primary ions scattered from the surface of the target and transmits ions having a preselected kinetic energy value to ion detector means. The ion surface analyzer incorporates a device for neutralizing the accumulative charging of the target so as to permit elemental surface analysis of electrically non-conductive specimens.
Canadian Pat. No. 1,058,772 to McKinney and Goff discloses an ion scattering spectrometer with two independent analyzers positioned adjacent to the material surface for determining the mass and kinetic parameters of ions scattered from the surface. By predetermining conditions within the analyzers so that only ions having certain and different characteristics can pass through each analyzer, a signal characteristic of surface atoms having a given mass can be generated.
Canadian Pat. No. 1,023,061 to Valentine and Goff discloses an improved technique for compositional depth profile analysis applicable to Ion Scattering Spectroscopy and Secondary Ion Mass Spectroscopy. A primary ion beam is caused to traverse and to impinge on a predetermined region of the sample surface, whereupon atoms on the surface within that region are sputtered from the surface. Ions, indicative of the surface region, as have a given mass are transmitted and detected.
Canadian Pat. No. 1,118,913 to Colby and Hull discloses a method of and apparatus for elemental analysis of solids by mass spectrometry, wherein the initial kinetic energy spread of the ions from the solid sample is relatively low, permitting simplification of the mass analyzer which need no longer be of the energy focussing type.
These patents have been presented herein only as background for the present invention.